In recent years, efforts have been made in various fields to utilize water-swelling polymers having a structure of crosslinked water-soluble polymer by taking advantage of the liquid absorbency and retainment thereof. The water-swelling polymers are studied for exploitation as hygienic goods such as disposable diapers, sanitary goods, contact lenses, cosmetics, paints, adhesive agents, water-stopping agents, soil-improving agents, etc. and also in the medical field for control of releasing of pharmaceuticals.
The water-swelling polymers, that are crosslinked polymers, are manufactured with various methods. A well-known, frequently used, industrial method is to, upon addition polymerization of a monomer of a vinyl compound, or vinyl compounds, by a radical polymerization or other method, introduce crosslinking points into the polymer chain by copolymerizing the vinyl compound(s) with a compound (i.e. a crosslinking agent) containing a plurality of functional groups, such as vinyl groups, that are copolymerizable with the vinyl compound(s).
Well-known examples of the water-swelling polymers obtained by the above process include polyacrylic acid liquid-absorbing resins and polyvinylacetamide liquid-absorbing resins (see Japanese Laid-Open Patent Applications No. 4-230250/1992 (Tokukaihei 4-230250) and No. 8-59743/1996 (Tokukaihei 8-59743).
However, the above-mentioned conventional liquid-absorbing resins hardly offer satisfactory liquid absorbency, especially, for electrolyte solutions containing a large proportion of salt and other substances, and therefore are limited in application when used in a field where an electrolyte solution containing various water-soluble substances needs to be absorbed and retained in a controlled manner. In addition, the polyacrylic acid liquid-absorbing resins dissolve and release a relatively large amount of soluble components when in contact with an aqueous liquid, causing safety and resistance problems.
Meanwhile, efforts have been made in various fields also to utilize liquid-absorbing resins having absorbed an organic solvent containing an electrolyte as an electrolyte solution by taking advantage of the liquid absorbency and retainment. A prospective field for such liquid-absorbing resins is thin, large capacity electric cells incorporated in mobile phones and notebook type personal computers which have seen large increases in the number of users in recent years. However, the electric cell employing an electrolyte solution is difficult to fabricate in a thin shape, because it needs to be completely sealed with, for example, a metal canister to prevent leakage of the electrolyte solution, but the metal container is difficult to fabricate in such a thin shape. Researches are being conducted to utilize a gel composition as a high polymer solid electrolyte in lieu of an electrolyte solution, the gel composition resulting from absorption of an organic solvent containing an electrolyte by a liquid-absorbing resin.
Examples of a high polymer solid electrolyte employed in such an electric cell include a high polymer solid electrolyte containing an organic solvent (electrolyte-containing organic solvent) produced by dissolving an electrolyte in a crosslinked polymer obtained by reacting an acryroyl compound having an urethane bond (see Japanese Laid-Open Patent Application No. 3-84807/1991 (Tokukaihei 3-84807), and a high polymer solid electrolyte composed by a crosslinked polymer containing an electrolyte, the crosslinked polymer being produced by reacting the mixture of a diacrylic acid ester and/or dimethacrylic acid ester of polypropylene glycol and a monoacrylic acid ester and/or monomethacrylic ester of a polyether (see Japanese Publication for Examined Application No. 8-32755/1996 (Tokukohei 8-32755).
None of the high polymer solid electrolytes disclosed by the laid-open patent applications above can retain a satisfactory amount of an organic solution having dissolved, or being capable of dissolving, an electrolyte (e.g., the ratio of the amount of a propylene carbonate retained by a crosslinked polymer to that of the crosslinked polymer is from 1:1 to less than 2:1). Those organic solvents should be absorbed and retained in larger quantities to improve properties of the electric cell.